High-intensity interval or HIIT can be described as a 8 to 12 cycles of high-intensity exercises followed by low to medium intensity exercise. When done properly, the high-intensity exercise should reach near maximum intensity of what a person is capable of doing. An entire HIIT session should last at least 15 minutes, but not more than 20 minutes.
Several studies have been done to determine the difference of impacts on our body and system of traditional endurance training versus high-intensity interval training.
Tabata and other studies
One of the most well-known study, published by Tabata, in 1996 (Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max) describes a regimen composed of 8 cycles of 20 seconds of ultra-intense exercise (170% of VO2max) followed by a 10 seconds rest, for a total of 4 minutes. Tabata compared subjects training with a 5 days x 20 minutes HIIT sessions and subjects using longer steady state cardio training (5 days x 60 minutes).
Other studies have used variations and slightly different approaches , such as 60 sec of of intense exercise (95% of VO2max) followed by 75 seconds of rest, repeated 8 to 12 times, or a 60 seconds of high intensity exercise followed by 1 minute of low intensity, repeated 6 to 8 times.
All the studies came with similar conclusions: subjects using the HIIT training were able to obtain better gains, using shorter training sessions, when compared to subjects using longer steady state cardio training. For example, in the Tabata study, subjects using HIIT rather than steady state cardio finished with a higher VO2max and had increased anaerobic capacity.
Even when doing nothing, your body need energy to function. The number of calories used if you were going to stay in bed all day doing nothing is known as your resting metabolic rate, or basal metabolic rate. HIIT has been shown to increase that resting metabolic rate up to 24 hours post-exercise (King J., A comparison of the effects of interval training vs. continuous training on weight loss and body composition in obese pre-menopausal women), which basically means that, in the 24 hours following your 20 minutes of HIIT session, your body keeps burning extra calories.
Although there are some controversy about the topic, many studies have shown that HIIT also helps reduce body fat (King J. A comparison of the effects of interval training vs. continuous training on weight loss and body composition in obese pre-menopausal women; Tremblay and al. Impact of exercise intensity on body fatness and skeletal muscle metabolism; Boutcher SH, High-intensity intermittent exercise and fat loss.). It is well-known that the proportion of fat used decreases and the proportion of carbohydrates used increases with increasing exercise intensity. This is why, for many years, the recommended way to reduce body fat was low-to-moderate intensity exercices, such as jogging. However, in the past two decades, many studies have found significant fat loss following a high-intensity exercise training than a low-to-moderate exercise training method.
The are many theories that explain this. First, many suggest that the source of fuel is irrelevant to the ending result. Most trainers never took in account the impact of the post-exercise metabolism. As a matter of fact, high-intensity lead to a significantly higher post-exercise energy expenditure. Other studies show a higher use of fatty acids during recovery and a smaller appetite following high-intensity training. Other theories imply more complexed biology, such as an increase in acetyl-CoA carboxylase inactivation, resulting in more free-fatty acid oxidation (Tremblay and al. Impact of exercise intensity on body fatness and skeletal muscle metabolism.).
Marathon Runners vs Sprinters
Still undecided on whether to train HIIT or steady state cardio? Observe the difference between the marathon runner and the sprinter.
You probably already noticed that sprinters have a much leaner and muscular look compared to marathon runners, who have tiny frames. Now ask yourself who has a better body.
In other words, HIIT is a great way to build up endurance and speed, to decrease body fat and to increase VO2 max and this, by drastically cutting down on cardio time.
1. Tabata I, Nishimura K, Kouzaki M, et al. (1996). “Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max”. Med Sci Sports Exerc 28 (10): 1327–30. doi:10.1097/00005768-199610000-00018. PMID 8897392.
2. King, J. (2001) “A comparison of the effects of interval training vs. continuous training on weight loss and body composition in obese pre-menopausal women”
3. Tremblay A, Simoneau JA, Bouchard C. (1994) “Impact of exercise intensity on body fatness and skeletal muscle metabolism.” Metabolism. Jul;43(7):814-8. PMID: 8028502
4. Boutcher SH. (2011) “High-intensity intermittent exercise and fat loss.” J Obes. 2011;2011:868305. Epub 2010 Nov 24., PMID: 21113312